1. Field of the Invention
This invention relates generally to bonding a propylene polymer layer to an enamel coated metal surface and more particularly to effecting a seal of controlled detachability between the bonded surfaces.
2. Prior Art
Easy opening containers are known to the art. These containers are generally formed of metal and are provided with a pour opening. The pour opening generally occupies only a portion of the end panel of the container. Heretofore, the pour opening has generally been formed by scoring to define a tear strip. A pull tab is attached to the tear strip, and upon the application of a force, the pull tab is operative to separate the tear strip along the weakening line from the panel.
Although easy opening containers have been readily accepted by the public, deficiencies still remain in this type of container. One of these deficiencies is that the removable tear strip which is torn from the can end in the opening of the can has sharp edges and, when thrown on the ground or otherwise improperly disposed of, remains as a nuisance which presents a cutting hazard to the public.
It has been proposed U.S. Pat. Nos. 3,292,828, 3,990,615 to replace the metal tear strip of easy opening containers with a plastic closure member which will eliminate the cutting hazard in that the removed portion does not have sharp edges on which a person may be cut. In these containers a tape strip is fabricated from a thermoplastic resin, such as polypropylene or a metal foil coated with the resin is detachably and sealably secured to the outer periphery of a preformed pour opening in the end panel of the container by bonding with a suitable adhesive. Container end panels used for pressurized beverages such as beer and soft drinks have been coated with a heat activatable adhesive layer such as a carboxylated polypropylene resin to provide a surface for bonding tape strips of the type described to the enamel coated surface of the container end panel to seal the preformed pour opening, e.g., U.S. Pat. No. 3,616,047, whereby the tape strip is heat sealed to and fixed about the periphery of the opening until a pulling force is applied thereto.
In U.S. Pat. No. 4,034,132 there is disclosed an easy opening container wherein the preformed opening of an enamel coated metal end panel is sealed with a tape strip provided with a heat activatable propylene polymer bonding layer which is heat sealed to the enamel coated end panel, the enamel coating having incorporated therein a bond promoting concentration of a carboxylated polypropylene resin.
Although the method disclosed in U.S. Pat. No. 4,034,132 is highly effective for bonding the tape strip to the can end, it has been determined upon further experimentation that if, during the application of the enamel coating to the end panel, the applied coating is overbaked during the curing and hardening step involved in the coating application, a wide variance in the strength of the heat seal bond or peel strength of the tape strip is encountered. For example, after an uncured epoxy resin based enamel coating formulation is applied to the metal end, the end is conveyed through an oven at 415.degree. F. for 10 minutes to harden the coating. If for any reason the exposure time of the end in the oven is extended substantially beyond the 10-minute period, the bondability or peel strength of the tape strip subsequently applied to the end has been found to vary over wide ranges and frequently will be lowered or raised to values which are unacceptable for commercial use. A commercial acceptable peel value when measured using a standardized test is in the range of 4 to 9 pounds per lineal inch (PLI). If the peel value of a tape strip used to seal containers in which carbonated beverages are packaged has a peel value below 4 PLI, the internal pressure generated by the carbonated beverage within the container will cause leakage of the product through the pour opening at the seal area. If the peel value is in excess of 9 psi, the force required by the consumer to effect the removal of the tape strip is too high for consumer acceptance.
In order for containers provided with the above discussed tape strip sealed end to obtain rapid consumer acceptance, it is necessary that the tape strip consistently and properly seal the pour opening in the container end, and when removal of the closure is attempted, the closure be readily separable from the can end.